The present invention relates to a power supply apparatus suitable for being used, for example, in electronic appliances which have an extensive range of an input commercial AC voltage.
Power source voltages used all over the world are roughly divided into a 100 V area and a 200 V area. Therefore, for an electronic appliance shipped without any specified destination, there is such a need that the appliance operates normally irrespective of which one of the power source voltages is applied thereto. However, if the power source apparatus is so structured, for example, by one AC-DC converter, such a fear arises that losses will increase in parts of a switching element, a transformer, and the like depending on a system of the applied power source.
Therefore, in a power source apparatus which has been in use with such an electronic appliance, an arrangement shown, for example, in FIGS. 1 and 2 has been used in the art. That is, in an arrangement of FIG. 1, an AC input voltage from a commercial power source 100 and the like is rectified in full wave by a diode bridge circuit 101. After the rectified output is smoothed, a DC output voltage is obtained by using a plurality of DC-DC converters 102, 103. According to this arrangement, the total losses can be reduced by dividing a load into the plurality of DC-DC converters.
Also, in the arrangement shown in FIG. 2, the smoothing action is conducted by a series circuit of a couple of capacitors 104, 105 and at the same time, a connecting middle point of the capacitors 104, 105 is connected to one AC input terminal of the diode bridge circuit 101 through a switch 106. Then, the switch 106 is controlled depending on a voltage at the AC input terminal, so that rectification is changed over from a primary side to a double voltage rectification in a 100 V system and to the full wave rectification in a 200 V system, which makes voltages supplied to the DC-DC converter equal so that an efficient control can be conducted by the single DC-DC converter 102.
With the system where a plurality of converters are provided as shown in FIG. 1, however, there is an increase in the number of parts compared with a system which is made up of one converter, and hence an increase in manufacturing costs. Also, with the rectification change-over system shown in FIG. 2, in case where, for example, a double voltage rectification is conducted in the 200 voltage system due to an abnormality, there is a possibility that a rectified output nearly approaches 800 V at the largest necessitating such counter-measures as to provide a specific safety device and the like for avoiding a damage to a circuit element and an accident.
That is, in an area where a state of the power source is unstable, for example, almost.+-.30% in fluctuations of a nominal power source voltage can ordinarily happen. In that case, there is a fear, for example, that a power source in the 200 V system is detected in error as that in the 100 V system, and in a changed over state to the double voltage rectification, if a voltage in the 200 V side rises rapidly, the rectified output possibly approaches 800 V at the largest.
Alternatively, although it is carried out that the control range is expanded by one converter, this inevitably requires switching elements and a transformer large in size, and brings about large-sizing of the whole appliances and worsening of transformation efficiency of the power source apparatus itself.